Cable fairings are structures attached to cables typically towed by marine vessels. They are designed to streamline the water flow around the cable and reduce normal drag and cable vibration caused by vortex shedding. Faired cables are used in applications such as underwater geophysical exploration and military reconnaissance operations, including towing buoys from ships, submarine detection, and deploying radio antennas from submarines. Rectangular ribbon fairings are the most popular type of fairings used to reduce vortex induced vibration and normal drag. Normal drag is the component of cable drag that is perpendicular to the cable axis. Conversely, tangential drag is the component of drag parallel to the cable axis.
Prior art FIG. 1 shows a commonly used ribbon fairing (100). The ribbon fairing (100) is a rectangular piece of material that is typically woven into the outer armor layer of a towed cable, as shown in prior art FIG. 2. The fairing is held by one or two strands of the outer layer of cable armor such that it is folded along its center line (105) and extend back in the wake of the cable. The fairings can also be molded onto a polymer jacket of a cable.
While ribbon fairings can be effective in reducing normal drag and cable strum, they introduce tangential drag when the tow angle is less than 90 degrees to the flow. This is because ribbon fairings naturally lie close to 90 degrees to the cable axis as shown in prior art FIG. 2. Consequently, as the tow angle decreases, more of the cross sectional area of the ribbon fairing is presented to the flow.
This increased cross sectional area becomes more problematic as the cable tension increases and the tow angle gets closer to horizontal. At angles of 30 degrees or less, the tangential drag of the rectangular ribbon fairing increases significantly. In fact, rectangular fairing tangential drag at these angles is typically ten times, or more, the tangential drag of the bare cable. Since each strip is held at its center (105) by the outer armor strands of the cable (210), the rectangular fairing warps or twists as the tow angle of the cable decreases and the ribbon tries to align with the flow. Consequently, instead of presenting a frontal area of t×w to the flow, where t is the ribbon thickness and w is the ribbon width, the rectangular fairing presents a frontal area on the order of w×w.
At more shallow cable angles, this tangential drag from the ribbon fairing can significantly increase the tow tension on the cable at the tow vessel. As a result, ribbon fairings are typically not used at angles shallower than 20-30 degrees. Therefore, there is a need for a fairing that reduces normal drag at shallow angles without introducing high levels of tangential drag.